Particulate matter (PM) emitted from engines and vehicles is defined by the United States Environmental Protection Agency as the materials collected from a dilute engine exhaust on a high efficiency (>99% particle removal) filter that is maintained at 47° C.±5° C. In addition, such filter medium is typically a polytetrafluroethylene (PTFE) membrane or PTFE coated glass fiber filter. With the increased stringency in emissions regulations, the PM filter weight gain during a typical heavy-duty or light-duty highway engine laboratory test dropped from a few milligrams for late 1980s engines to a few micrograms for 2007 engines and beyond.
With the reduction in PM emissions and PM collection on a filter during an engine and/or a vehicle test, filter sampling, handling and weighing has become important to the accuracy and reduced variability in filter measurement. One source that leads to measurement uncertainty is gas phase artifacts collection by the PM filter during sampling. Ideally, the filter should only collect airborne particles that amount to liquid droplets and/or solid particles and/or particles with adsorbed layers of particle phase hydrocarbons or sulfate species. However, in practice, during filter collection, the PM filter tends to adsorb materials from the gas phase (gas phase artifacts) significantly increasing the uncertainty in filter measurements. Some main sources of these gas phase artifacts are gas phase hydrocarbons and sulfuric acid that are present in engine exhaust.
Accordingly, when evaluating a selected engine exhaust for regulatory compliance, it is important to ensure that the determination of particulate matter presence is as accurate as possible and not adversely influenced by artifacts that compromise the ability to correctly determine particulate levels. Such need is even more pressing in the face of the relatively more stringent particulate matter levels for engine exhaust noted above.